The reader may be surprised that we end up, this application, with a perforated sheet having "4" holes at a spacing as great as 3.8" on center, and with only 0.08 holes/sq in. But it is important to realize that, in this case, we are NOT trying to achieve the maximum exposure of the sound absorptive blanket, as we did in the "transparency approach". In fact, that approach would be effective in this application only if we could afford a six-inch blanket of glass fiber, in order to get high apsorption efficiency at the low frequency of 120 Hz due to its thickness alone .

Instead, we are aiming at a combination of perforation pattern and absorber depth (h) that will encourage maximum air particle motion through the absorptive material at the frequency of interest, by deliberately creating a resonance at that frequency. (See the further discussion of the significance of material thickness for low-frequency absorption, in Appendix A).

Alternative combinations of plate thickness, hoie size and percent open area that would achieve the same resonance frequency are illustrated in two further examples.